Chore: add custom selectpanel event

[francinelucca]

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What are you trying to accomplish?

Adds a new "invokerClicked" custom event to SelectPanel before stopping propagation of original event so that consumers can subscribe to SelectPanel activation

Integration

N/A

List the issues that this change affects.

Closes https://github.com/github/primer/issues/6372

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

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Accessibility

• Fixes axe scan violation - This change fixes an existing axe scan violation.
• No new axe scan violation - This change does not introduce any new axe scan violations.
• New axe violation - This change introduces a new axe scan violation. Please describe why the violation cannot be resolved below.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Lookbook)
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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[changeset-bot]

🦋 Changeset detected

Latest commit: bd75545

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Patch

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[Copilot]

Pull request overview

Adds a new custom event around SelectPanel invoker activation so consumers can observe invoker interactions even though the original activation event is prevented/stopped from bubbling.

Changes:

• Dispatch a new custom event during SelectPanel invoker activation handling.
• Add system tests asserting the new event fires on click, keyboard activation, and bubbles.
• Add a changeset entry and apply a small string-quote/style tweak in a TreeView system test.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
app/components/primer/alpha/select_panel_element.ts	Dispatches a new custom event when the invoker is activated.
test/system/alpha/select_panel_test.rb	Adds system tests to validate the new invoker event behavior (click, keyboard, bubbling).
test/system/alpha/tree_view_test.rb	Minor string quoting/style adjustment.
.changeset/rare-jokes-scream.md	Documents the change as a patch release.

Comments suppressed due to low confidence (1)

app/components/primer/alpha/select_panel_element.ts:817

• This custom event fires for both mouse click and keyboard activation (Enter), so the name/comment "invokerClicked" is misleading. Consider renaming the event (and related test names) to reflect activation rather than a pointer click.

    // Let listeners observe the invoker was clicked
    this.invokerElement?.dispatchEvent(new CustomEvent('invokerClicked', {bubbles: true}))

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